With a rapid increase of communication services, 3rd Generation Partnership Project (3GPP) licensed spectrums become insufficient to provide higher network capacity. Therefore, the 3GPP proposes a concept of Long Term Evolution (LTE) Assisted Access (LAA), which uses unlicensed spectrums with the help of licensed spectrums. An LAA scheme is based on carrier aggregation to deploy LTE systems in unlicensed frequency bands.
The unlicensed spectrum can have two working modes. One is supplemental downlink (SDL), in which only downlink transmission subframes are included; the other is time-division duplex (TDD) mode, in which both uplink transmitting subframes and downlink transmitting subframes are included. SDL can only be used with the help of carrier aggregation technology. In addition to being used with the help of the carrier aggregation technology, the TDD mode can also be used with the help of Dual Connectivity (DC) or independently.
Existing schemes only address problems when the unlicensed spectrums and the LTE licensed spectrum work in a carrier aggregation manner, problems when the unlicensed spectrums and the LTE licensed spectrum work in a dual connectivity manner are not addressed. In many circumstances, a connection between a base station where an unlicensed spectrum is deployed and a base station where a licensed spectrum is located is not ideal, and only the dual connectivity manner can be used.
In the case of dual connectivity, a Secondary Evolved Node B (SeNB) needs a primary secondary cell (PSCell) to provide some functions of a primary cell (PCell), such as sending a Master Information Block (MIB). Information contained in the MIB mainly includes a System Frame Number (SFN), a downlink bandwidth, and Physical Hybrid ARQ Indicator Channel (PHICH) configurations. Only the SFN cannot be sent by the PCell. Furthermore, the MIB is transmitted in a period of 40 ms, and is repeatedly transmitted 4 times every 40 ms. The MIB is transmitted in subframe#0 of a radio frame with a SFN that is a multiple of 4 for the first time, and information same as in subframe#0 of the first radio frame is transmitted in subframe#0 of next three radio frames. Information transmitted within a next 40 ms may be different from information transmitted within a previous 40 ms.
In addition, on unlicensed spectrums, PCells working on unlicensed carriers, that is cells on unlicensed spectrums, can also be deployed to work independently (i.e., standalone), to implement communication control.
However, because it is required to use a Listening Before Talk (LBT) mechanism to occupy an unlicensed spectrum. If a channel is occupied by other devices, the MIB cannot be sent normally. As a result, users cannot receive the system frame of the MIB normally to perform a time synchronization, and the communication fails.